The present invention relates to a muffler for use in combination with an internal combustion engine, and more particularly to a muffler installed in an exhaust system of the engine in which thermal stresses created by a temperature differential between an inner pipe and an outer cylinder thereof, are decreased.
Generally, a conventional exhaust system comprises an exhaust pipe, through which the exhaust gas created by an engine flows, and a muffler for absorbing some of the sound waves associated with the exhaust gas. The conventional exhaust system is schematically represented by FIG. 1. A sub-muffler 1 is provided in a position between an exhaust manifold 5, which is connected to the engine, and a main muffler 6. The sub-muffler 1 primarily absorbs the high frequency components of the sound waves associated with the exhaust gas and further absorbs any resonance created in exhaust pipes 2, 3 and 4. FIGS. 2 and 3 disclose the two types of conventional sub-mufflers in existence, the resonance and expansion types. The resonance type muffler disclosed in FIG. 2, comprises an inner pipe 8, having a number of holes therein, and an outer cylinder 9. The exhaust gas flows into the muffler 9 through the pipe 8, and any associated exhaust noise is attenuated by resonance occurring within the chamber defined by the outer portion of the inner pipe 8 and the inner portion of the outer cylinder 9. The expansion type muffler disclosed in FIG. 3 comprises an inner pipe 10 and an outer cylinder 11. In the expansion type muffler shown in FIG. 3, the exhaust noise or sound of the exhaust gas is attenuated when the exhaust gas flowing through the inner pipe 10 expands into the chamber defined between an outer face of the inner pipe 10 and an inner face of the outer tube 11. According to these conventional sub-mufflers 1 and 2, the high frequency part of the exhaust noise is effectively attenuated or eliminated.
However, whenever a sub-muffler is placed in a conventional exhaust system, the flow resistance (back-pressure) of the system increases, thus reducing the engine's power and therefore lowers the combustion efficiency of the engine. Therefore, the conventional sub-mufflers are not sufficient to satisfy both the high frequency noise attenuation requirement and the low exhaust flow resistance requirement. In general, whenever the flow resistance in an exhaust system increases, the high frequency sound attenuation increases and conversely, when the flow resistance decreases, the high frequency sound attenuation also decreases. The afore-described sub-mufflers cannot achieve the desirable effects of low flow resistance and high sound attenuation.
To obviate the afore-mentioned disadvantages of the previously discussed conventional sub-mufflers, a sub-muffler as disclosed in FIG. 4, and which is described in Japanese laid open patent publication No. Sho 49-64738 has been introduced. According to the sub-muffler disclosed in FIG. 4, a sound absorbing fiber 12, consisting of glass wool, is filled into a space defined between an inner pipe 8 and an outer cylinder 9. Use of the sound absorbing fiber results in attenuation of high frequency exhaust noise without significantly increasing the back-pressure of the entire exhaust system. However, when the sound absorbing fiber is filled into the sub-muffler, it results in an increase in the temperature differential between the inner pipe 8 and the outer cylinder 9, resulting in a considerable difference in the amount of thermal expansion between the inner pipe 8 and the outer cylinder 9. Therefore, thermal stresses arise in the muffler assembly which may result in structural defects because there is no means provided to absorb the generated thermal stresses.
In order to obviate the problem of these generated thermal stresses, a sub-muffler, as disclosed in FIG. 5, has been conventionally employed. The structure of the sub-muffler disclosed in FIG. 5 combines features of both the resonance type sub-muffler and the expansion type sub-muffler. The inside space of sub-muffler 1 is filled with a sound absorbing fiber 12, and by a chamber 13 defined by a pair of separators 14, which aids in maintaining a small temperature differential between the inner pipe 10 and the outer cylinder 11. According to the sub-muffler disclosed in FIG. 5, there is no provision for any sound absorbing fiber 12 within the chamber 13. Consequently, the silencing effect attributed to attenuation of exhaust gas noise is not satisfactory. Further, the separators 14 are not fixed to the outer cylinder 11, thereby providing for absorbtion of any thermal stresses generated by the difference in temperature between the inner pipe 10 and the outer cylinder 11. However, when the muffler is subjected to externally created stresses, such as those arising from a vehicle traversing a rough road, the separators 14 slide relative to the outer cylinder 11, thus reducing the service expectancy of the sub-muffler because of the repeated oscillations.